#include <bits/stdc++.h>
using namespace std;
// macros
typedef long long ll;
typedef long double ld;
typedef pair<int, int> ii;
typedef pair<ll, ll> lll;
typedef tuple<int, int, int> iii;
typedef vector<int> vi;
typedef vector<ii> vii;
typedef vector<iii> viii;
typedef vector<ll> vll;
typedef vector<lll> vlll;
#define REP(a,b,c) for(int a=int(b); a<int(c); a++)
#define RE(a,c) REP(a,0,c)
#define RE1(a,c) REP(a,1,c+1)
#define REI(a,b,c) REP(a,b,c+1)
#define REV(a,b,c) for(int a=int(c-1); a>=int(b); a--)
#define FOR(a,b) for(auto& a : b)
#define all(a) a.begin(), a.end()
#define INF 1e9
#define EPS 1e-9
#define pb push_back
#define popb pop_back
#define fi first
#define se second
#define sz size()
mt19937_64 rng(chrono::steady_clock::now().time_since_epoch().count());
// input
template<class T> void IN(T& x) {cin >> x;}
template<class H, class... T> void IN(H& h, T&... t) {IN(h); IN(t...); }
// output
template<class T1, class T2> void OUT(const pair<T1,T2>& x);
template<class T> void OUT(const vector<T>& x);
template<class T> void OUT(const T& x) {cout << x;}
template<class H, class... T> void OUT(const H& h, const T&... t) {OUT(h); OUT(t...); }
template<class T1, class T2> void OUT(const pair<T1,T2>& x) {OUT(x.fi,' ',x.se);}
template<class T> void OUT(const vector<T>& x) {RE(i,x.size()) OUT(i==0?"":" ",x[i]);}
template<class... T> void OUTL(const T&... t) {OUT(t..., "\n"); }
template<class H> void OUTLS(const H& h) {OUTL(h); }
template<class H, class... T> void OUTLS(const H& h, const T&... t) {OUT(h,' '); OUTLS(t...); }
//===================//
// Added libraries //
//===================//
//===================//
//end added libraries//
//===================//
void program();
int main() {
ios_base::sync_with_stdio(false);
cin.tie(NULL);
cout.tie(NULL);
program();
}
//===================//
// begin program //
//===================//
const int MX = 31000;
int n, m;
int b[MX], p[MX];
int d[MX];
set<int> dif[MX];
vii adj[MX];
void program() {
IN(n,m);
RE(i,m) IN(b[i],p[i]);
RE(i,m) dif[b[i]].insert(p[i]);
// create graph
RE(i,n) {
FOR(del,dif[i]) {
int x, dist;
// left
x=i; dist = 0;
while(1) {
x -= del; dist++;
if(x < 0) break;
adj[i].pb({x,dist});
if(dif[x].count(del)) break;
}
// right
x=i; dist = 0;
while(1) {
x += del; dist++;
if(x >= n) break;
adj[i].pb({x,dist});
if(dif[x].count(del)) break;
}
}
}
// dijkstra
RE(i,n) d[i] = INF;
priority_queue<ii> pq;
pq.push({0,b[0]}); d[b[0]] = 0;
while(!pq.empty()) {
ii p = pq.top(); pq.pop();
if(d[p.fi] != p.se) break;
FOR(v,adj[p.fi]) {
if(d[v.fi] > p.se+v.se) {
d[v.fi] = p.se+v.se;
pq.push({v.fi, d[v.fi]});
}
}
}
int ans = d[b[1]];
if(ans == INF) OUTL(-1);
else OUTL(ans);
}
